Traditionally cotton thread was preferred for piecing because the polyester threads were stronger and therefore could cut through the weaker cotton fibers. However, nowadays many polyester threads are just as weak (or weaker) than cotton threads. In any case, thread is not just thread; brand matters as well as type.

guess im bad...i use cotton, rayon, polyester, silk, wool threads, and some blends...cotton/poly...silk/rayon ect...i have my favorites for different projects and think that you can use any thread you like as long as it does what you want it to do...